In the oil and gas industry, open-ended tubular members installed axially into the earth are used for a variety of purposes. For example, open-ended tubular members are frequently used as foundation piles to support the weight of an offshore structure and to resist environmental loads applied to the structure. Open-ended tubular members are also used as well conductors to facilitate the drilling of wells from an offshore platform. Typically, these well conductors extend from the deck of the offshore platform downwardly through the body of water and into the earth to a point which may be located several hundred feet below the bottom of the body of water. Well conductors are required to resist their own weight and, typically, the weight of the first string of well casing (which may be in excess of 300,000 pounds) until it has been cemented to the formation. Other uses of open-ended tubular members installed axially into the earth will be known to those skilled in the art.
Typically, the objective is to install the open-ended tubular member into the earth a distance (known as the target penetration) which is sufficient to mobilize the required load carrying capacity. The load carrying capacity of a tubular member is the sum of the end bearing resistance of the lower end (i.e., the "toe") of the tubular member and the frictional resistance along the outside of the tubular member.
If the tubular member is a well conductor, another objective is to preclude soil fracture during subsequent drilling operations. The ability of the subsurface soils to withstand fracture is known as "fracture integrity." Fracture integrity may be either local or global. Local fracture integrity refers to the ability to withstand fractures along the interface between the conductor and the surrounding soils (also known as "piping"). Global fracture integrity refers to the ability of the soils to withstand fractures at some distance from the wall of the conductor or below the toe of the conductor.
Traditionally, the installation of open-ended tubular members has been accomplished by impact driving in which repeated blows from a large hammer are used to literally pound the open-ended tubular member into the earth. More recently, resonant or vibratory driving techniques have been developed.
Oftentimes, an open-ended tubular member can be installed to the target penetration by impact, resonant, or vibratory driving alone. However, under certain conditions, such as in sandy soils or interbedded sands and clays, high soil resistance that develops along the wall and below the toe of the tubular member may result in a premature driving refusal (i.e., the situation where continued driving efforts do not result in any appreciable advances of the tubular member prior to achieving target penetration). In these situations it may not be possible to install the open-ended tubular member to the target penetration by driving alone.
When an open-ended tubular member is driven into the earth, a soil column will form within the tubular member. As the tubular member moves past the soil column, a frictional resistance develops between the inside wall of the tubular member and the soil column. As this friction increases, the amount of soil entering the tubular member decreases (known as "partial plugging") until the tubular member fully plugs, which occurs when the skin friction along the inside wall of the tubular member becomes equal to or greater than the toe bearing capacity of the cross-sectional area of the tubular member. Thereafter, the tubular member advances, if at all, in the same manner as a closed-ended member.
One consequence of a tubular member penetrating in either a partially or fully plugged condition is that some or most of the soil immediately below the toe of the tubular member, the amount depending on the degree of plugging, is displaced into the surrounding soils. In granular soils, and to a lesser extent in cohesive soils, this displacement produces densification or compaction of the soils immediately adjacent to and below the toe of the tubular member, resulting in increases in both normal and shear intergranular stresses and, accordingly, an increase in the fracture integrity of the surrounding soils. As the tubular member is penetrated into the zone of densified soils, the high intergranular stresses in the soils, which are above the ambient values of the formation, produce high lateral stresses on the outside wall of the tubular member. These high lateral stresses increase the shear capacity, or skin friction, on the outside wall of the tubular member by simple Coulomb friction. Increased toe bearing capacity also results and together with the increased frictional capacity may produce driving refusal prior to achieving the target penetration.
One apparatus that has been proposed for solving the premature refusal problem is disclosed in U.S. Pat. No. 4,702,325 issued Oct. 27, 1987, to James Hipp. The Hipp apparatus consists primarily of an external reciprocal impact driving means supported atop the tubular member and a rotatable drilling means supported within the tubular member for removing the soil plug. In order to protect the drilling means from the adverse effects of the impact driving means, the drilling means is supported within the tubular member at the bottom portion thereof near the point of least energy absorption and rebound. The driving and drilling operations may be performed simultaneously or sequentially.
In the Hipp apparatus, a special landing nipple must be provided on the lower end of the tubular member. This landing nipple includes an internal stop shoulder that cooperates with a lock-in notch located on the lower end of the drill string to support the drilling means. The drill bit is positioned at the toe of the tubular member so that the entire soil plug is removed, and the rates of the driving and drilling operations are adjusted so that the drill bit remains in position at the toe of the tubular member.
Experience has shown that completely removing the soil plug from inside the tubular member, as proposed by Hipp, may permit the tubular member to be installed to the target penetration. When using this technique, however, the target penetration is often achieved at the expense of failing to mobilize the desired load carrying capacity of the tubular member and/or compromising the fracture integrity of the surrounding soils. Failure to mobilize the desired load carrying capacity of a tubular member means that the installed tubular may not be fit for its intended purpose since it may not be able to resist the applied loads. Further, if the tubular member is a well conductor, compromising the fracture integrity of the surrounding soils can lead to lost returns and, potentially, to loss of the well during subsequent drilling operations.
Thus, a need exists for an apparatus and a method which permit open-ended tubular members to be installed to the target penetration in all types of soils without compromising the load carrying capacity of the tubular member or the fracture integrity of the surrounding soils. The present invention satisfies this need.